The purpose of this fork is to make the support available for those who were asking for it, and to test it while it's not integrated into the upstream project.
These changes were already integrated in the upstream project, meaning that this fork is officially dead.
Please, check the gpsbabel site and documentation.
As a simple example, to convert a GTM file into a GPX file, do something like:
gpsbabel -i gtm -f myfile.gtm -o gpx -F myfile.gpx
To convert a GPX file into a GTM file, it's just a matter of doing the opposite:
gpsbabel -i gpx -f myfile.gpx -o gtm -F myfile.gtm
The patch was made by Gustavo Niemeyer <email@example.com>